Project Abstract We have recently shown that sAPP? directly inhibits ?-secretase (BACE1) proteolysis of APP, thereby reducing A? production. In addition, our most recent studies show that sAPP? reduces Alzheimer?s disease (AD)-related tau-hyper-phosphorylation by inhibition of GSK3?. Moreover, while multiple low dose infusions of human umbilical cord blood-derived monocytes (CB-M) reduce AD pathology and behavioral impairment in PSAPP mice, in part via phagocytosis of A?, aged monocytes show impaired A? phagocytosis, which can be reversed by sAPP?. These results suggest that CB-M have their own ?-secretase or activate endogenous ?- secretase in the PSAPP mice. Thus, restoration of sAPP? levels in the brain by shifting the amyloidogenic towards the non-amyloidogenic (?-secretase) pathway could ameliorate AD-related amyloid and tau pathology, neuronal loss and cognitive impairment. An increase in ?-secretase activity, therefore, is an attractive strategy for treatment of AD. Several years after descriptive reports on ?-secretase, the responsible enzymes have been identified as members of the family of A Disintegrin And Metalloproteinase (ADAM). Three of these membrane-anchored zinc-dependent metalloproteinases, ADAM10, ADAM17 and ADAM9, possess ?- secretase activity. However, during the past years, numerous other substrates have been linked to all three sheddases, some of which could promote AD pathogenesis via activation of cytokines or chemokines. Indeed ADAM17 is also known as the TNF? converting enzyme (TACE), which releases TNF?, a proinflammatory cytokine. It has also been reported that CD40L is released from T-cells via ADAM10 and 17 upon CD40 ligation. Furthermore, ADAM10 and ADAM17 are not APP-specific, cleave various substrates and have been associated with tumorigenesis and tumor progression. We have most recently found that human umbilical cord blood serum (CBS) markedly enhance APP ?- cleavage and thereby increase sAPP? production in CHO cells expressing wild-type human APP. In addition, CBS selectively mediated the ?-secretase cleavage of neuron-specific, human recombinant full-length APP695 (fAPP695) in a cell-free system, independent from TACE and ADAM10. We partially purified this ?-secretase using DEAE, preparative-grade size exclusion and further ion-exchange chromatography and the present purified fraction (?CBS Frac.) dose-dependently cleaves fAPP695, also independent from TACE and ADAM10. Importantly, ?CBS Frac. does not cleave precursor pro-inflammatory cytokines, including IL-6 and TNF?. Furthermore, treatment of 3XTg-AD mice with ?CBS Frac by i.c.v. injection markedly increases cerebral sAPP? levels together with significant decreases in cerebral A? production and abnormal tauThr231 phosphorylation. The ultimate goal of this proposal will be to further characterize the effects of ?CBS Frac on AD pathology and behavioral impairment in 3XTg-AD mice, which display both A? and tau pathology, and ultimately identify and characterize this novel and APP specific ?-secretase.